Great idea. I could use a Size 7 ammo rack for my Cutter.
I'd have to dig up the old thread...but that's essentially 'almost' 200% more ammo, if I recall.
I think I had it worked up where a size 4 module basically doubles (so 100%) of your ammo, but each size only added like 25% ammo (so not scaling like cargo racks, which double). The general concept was built with wing support in mind (restocking limpets) such that a wing could remain in the field longer.
Following the rules of other support limpets, they can only be used on ships not currently in combat (so you can't just park a Type-9 on the edge and farm away) forcing you to finish the encounter or go to super-cruise and drop again. Self-stocking would be possible, similar to an AFMU, but you're giving up module slots for longevity in the field and the module can't be synthesized into so once it's out, it's out.
That's the basic concept - mostly intended to round out the refuel/repair/restock triangle and finalize the role of a support ship in a wing, but could be used by a solo pilot too just to improve staying power in the field, such as in combat zones. I personally don't subscribe to the 'use lasers' philosophy for ammo - that implies lasers are meant for that, when they aren't. It also assumes we like grinding mats for synthesis when most of us don't like using the darn thing in combat to begin with!
Anyhow, like I said, just an idea mirroring already existing features.